Industrial waste liquid may contain formaldehyde or methanol. Such waste liquid is discharged after it is treated to make the concentration of formaldehyde fit a waste liquid standard. As a method of treating formaldehyde or methanol in the waste liquid, the activated sludge method is used, in many cases.
In the activated sludge method, formaldehyde: a material originally harmful to living things, is treated with microorganisms, and in the treatment, a stage called “acclimatization” raises the concentration of formaldehyde gradually, causing the microorganisms to gain resistance against formaldehyde, and simultaneously causing the microorganisms to decompose formaldehyde, whereby waste liquid having a target formaldehyde concentration is treated. During acclimatization, it is possible to treat only formaldehyde-containing waste liquid having a concentration of a fraction of several tens or several hundreds of a target formaldehyde concentration, or having a smaller concentration. Thus, it is necessary to wait several months before treatment of waste liquid having the target formaldehyde concentration can be performed.
JP-A-11-19685 (“JP-A” means unexamined published Japanese patent application), JP-A-11-19686, JP-A-9-253696, and JP-A-7-232178 disclose methods of treating formaldehyde-containing waste liquid, in which the period of acclimatization can be made short or is not required. JP-A-11-19685 and JP-A-11-19686 describe methods of using a specific microorganism capable of decomposing high-concentration formaldehyde. Further, JP-A-9-253696 and JP-A-7-232178 describe methods of decomposing formaldehyde without using any activated sludge. The methods described in JP-A-9-253696 and JP-A-7-232178, which are different from any activated sludge method, require the setting up of facilities and catalysts, to raise the temperature and pressure of waste liquid to be treated.
On the other hand, as to facilities for waste liquid treatment in the activated sludge method, JP-A-8-24885 discloses a method of using a microfiltration membrane filter, thereby omitting a sedimentation tank for sludge, which is conventionally necessary in the activated sludge method, and saving on facility-installation space.
Incidentally, in recent years, the demand for water has been increasing, based on a rising population, and the amount of water that can be used per person has been increasing, based on improvement in the level of living, while new development of water (fresh water) resources has not been pursued. Further, the demand for water that can be used to produce food has also been increasing, from the rise in population. As a result, distribution of water resources between daily life water and agricultural water has been coming to the fore as a new problem.
To obtain fresh water by seawater desalting, it is essential that a place therefor is geographically near the sea. Further, to remove salts and the like from seawater, it is necessary to consume energy of a heat source or the like.
Meanwhile, waste water or sullage discharged (formed) following reactions in chemical plants or the like, is treated for reuse as a raw material water, boiler feed water, cooling water, or the like in plants, industrial complexes, or the like. In general, however, water generated by chemical reaction contains a variety of salts, reaction products, or compounds harmful to the human body, in many cases. In particular, salts are easily dissolved in water; and, metal components of pipes used in plant, catalysts used in the reaction, and the like may be present as salts in aqueous solution. In many cases, it is difficult to remove such salts by filtration or some other operation, or to remove only the salts selectively and effectively. For this reason, great cost is necessary to treat waste liquid to such a level that the resultant waste liquid can be used as life water or agricultural water. Thus, in general, the treatment has not been conducted.